Schedule of Changes in Fair Value of Level 3 Contingent Consideration (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 29, 2019	Sep. 30, 2018	Sep. 29, 2019	Sep. 30, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period		$ 26,847	$ 60,914	$ 70,543	$ 45,102
Acquisition of MiR	[1]	(7,759)	(768)	(16,460)	(9,236)
Foreign currency impact		(1,008)	796	(1,413)	(1,770)
Payments	[2]			(34,590)	(24,553)
Fair value adjustment					51,399
Balance at end of period		$ 18,080	$ 60,942	$ 18,080	$ 60,942

[1] In the three and nine months ended September 29, 2019, the fair value of contingent consideration for the earn-out in connection with the acquisition of MiR was decreased by $7.8 million and $16.5 million, respectively, primarily due to a decrease in the forecasted revenue, partially offset by impact from the modification, in the three months ended September 29, 2019, of the earn-out structure. In the three and nine months ended September 30, 2018, the fair value of contingent consideration for the earn-out in connection with the acquisition of Universal Robots A/S (“Universal Robots”) was decreased by $0.8 million and $9.2 million, respectively, primarily due to a decrease in forecasted revenue.

[2] In the nine months ended September 29, 2019, Teradyne paid $30.8 million and $3.8 million of contingent consideration for the earn-outs in connection with the acquisition of MiR and Universal Robots, respectively. In the nine months ended September 30, 2018, Teradyne paid $24.6 million of contingent consideration for the earn-out in connection with the acquisition of Universal Robots.